Your 3rd-party tool like DocuSave or Kwiktag are likely to store their data in a dedicated file system or database (KwikTag uses SQL Express to manage their documents). A procurement system like eRequester pro stores attachments in a file/folder structure per company.
As an example, I've a customer that has 50+ GP companies.. not all are using Doc Attach, but here are a few data samples to compare:
about 2k docs = 3GB of data (just for the coAttachmentItems table which is the blob storage)
about 3,1k docs = 1.7GB of data (depends vastly on doc type, but PDF & XLS mostly)
about 3.3k docs = 890Mb
As you can see, just based on those 3 samples, it can't really be calculated how much space you're going to waste in each company. Setting a max size default value else then 1GB might sure help to keep size growth in control:
Administration >> Setup >> Company >> Document Attachment Setup